Magnus--
Gregory could not possibly have thought he was doing the ethically correct thing when murdered that widow, or his wife Minerva.
Oh yes he could. It would be impossible for you or me, but not for him. It's amazing the kinds of total contradictions people can contain in their minds. That's where the idea for Orwell's "doublethink" began.
Minerva was a trial for him, someone whose constant suspicions were making it hard for him to have the serenity to do his work for the Lord properly and well.
There is a scene between Evan Hanley and G Trask, with ONLY the two of them in the room... and it's a conversation in which Trask has just blackmailed Hanley into agreeing to help Trask murder Minerva. Trask continues to speak as a "man of God", spouting righteous ideas and morals, even in the middle of this conversation. Remember, he's not talking to anyone except the man he's just arranged a murder with, so he doesn't need to lie to impress Evan with his "goodness"... therefore, Trask must really believe this about himself.
He knows on some very deep-down level it's wrong, but he's a man of God and the usual rules are flexible for special people like him. Rationalization becomes very easy after you've been practising a few decades.
Trask #1 is a similar hypocrite, lying to himself so that he can "honestly" lie through his teeth to others. It's about ego and power. As I said, if there were anything virtuous in him, he'd have some humility and question his own assumptions and actions, and take much more care before accusing anyone. He also would not enjoy the job so much. He's about to explode with pride as he accuses and torments and ties governesses to trees, etc..